Study On High Cycle Fatigue And Thermal Fatigue Properties And Damage Mechanism Of Vermicular Graphite Cast Iron In Cylinder Head

As a kind of thin-walled cast iron,cylinder head of internal combustion engine has complex structure.With the rapid development of economy,the demand for internal combustion engine with high power density is increasing.To enhance the power density,proper mechanical property and thermal stability of the cylinder head are required.During service the cylinder head is generally subjected to cyclic loading due to shocking of the high-temperature and high-pressure gas in the engine.Besides,temperature variation between cooling water and high-temperature gases also exists.Therefore,mechanical property and thermal stability of the cylinder head need to be considered.Moreover,the production cost of the internal combustion engine must be considered.Therefore,the material with high strength,small volume,light weight and good thermal stability become the criteria for the selection of the cylinder head material.Vermicular cast iron,as a kind of new type of cylinder head material,is widely used due to its good thermal stability and high strength.Generally,the mechanical property and damage mechanism of vermicular cast iron are particularly related to graphite and matrix structure.Base on the working conditions of the internal combustion engine cylinder head,several investigations have been carried out on the RuT450,including tensile test,high-cycle fatigue test and thermal fatigue experiment at different temperature range.The relationship between the microstructure of material and damage mechanism under tensile,fatigue and thermal fatigue was studied to provide a theory base for optimizing the investigation on the cylinder head.The main work can be summarized as follows:(1)The elongation of the RuT450 decreases first and then decreases with the increase of the testing temperature.The strength increases due to the existence of dynamic strain aging at the testing temperature of 400?,whereas the plasticity decreases.The increase of elongation at 500? is due to the occurrence of grain-boundary sliding.(2)The failure of RuT450 is mainly due to cleavage fracture,The fraction of cleavage fracture on the fracture surface of the specimen decreases with the increase of testing temperature.Tensile strength of the RuT450 increases with the increase of pearlite content.Strength of the pearlite to the material is low due to its low content and dispersed distribution.(3)High-cycle fatigue experiments were performed at 25?,400? and 500? for the RuT450.The failure of the materials was related to the vermicular graphite.The fatigue strength decreases with the increase of the temperature,and the decrease is slow first and then rapid.The dispersity of the S-N curves decreases with the increase of the temperature.(4)Thermal fatigue experiments were performed for the RuT450 at the temperature range of 25-450?,25-550?,25-650? and 25-750?.The length and the width of the main crack are related to the upper testing temperature.The length and the width of thermal fatigue cracks significantly increased in the temperature range of 25-750?.The length of thermal fatigue crack exhibits linear relationship with the number of thermal cycles.The propagation of thermal fatigue crack is related to the end orientation of the graphite phase...